A comprehensive ‘atlas’ of the toxic immune cells that can damage the brain has been identified by researchers at the Gladstone Institutes (CA, USA). Their study, which has recently been published in Nature Immunology, describes the use of a technique known as Tox-seq to develop the molecular profile, which they report has been successful in identifying a potential new drug target for multiple sclerosis (MS) and possibly other diseases.
Tox-seq is a method that integrates single-cell RNA-sequencing technology with selective labelling of cells that produce oxidative stress to reveal which genes are ‘on’ or ‘off’. Within their study, the team developed this method to understand how oxidative stress-producing immune cells inflict damage on the CNS.
In a mouse model of MS, Tox-seq was applied to immune cells from the CNS to reveal which subtypes of the cells produce reactive oxygen species. The researchers also examined which subtypes expressed a distinct gene signature that was associated with oxidative stress.
To their surprise, Tox-seq revealed that only one subgroup of microglia caused oxidative stress, alongside the subgroups of immune cells from the periphery that entered the brain. They also discovered that the gene expression signature of those microglia in the mouse model matched the pattern observed in cells that had previously been suspected of causing damage in progressive MS patients.
In addition to this, Tox-seq indicated that the pattern of gene expression associated with oxidative stress in the mouse cells included those involved in coagulation. Many neurodegenerative diseases have pointed towards leaky blood vessels in the brain, and previous research from the team has also established that leakage of the coagulation factor fibrinogen into the brain activates microglia, which subsequently promotes production of reactive oxygen species.
Moreover, the Tox-seq data showed that toxic microglia express genes that induce coagulation. “This is the first time we have evidence that coagulation and oxidative stress are at work in the same immune cells in the brain. It’s a vicious cycle between the two processes,” stated Akassoglou.
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The investigators then explored how their Tox-seq results could be used to identify compounds that could combat oxidative stress in the CNS. Using high-throughput screening, they identified 128 compounds that could suppress activation of microglia induced by fibrin. However, their approach did not identify which of those compounds specifically affect oxidative stress.
“At this point, our solution was to computationally overlay the oxidative stress gene signature identified by Tox-seq with previously published drug-target pathways”, explained co-first author Jae Kyu Ryu (Gladstone Institutes). “This type of overlay had never been done before for oxidative stress pathways.”
By comparing the known targets of the hits from the screen with Tox-seq data, the scientists were able to identify drugs relevant to toxic immune pathways. One of their hits included a cancer drug termed acivicin, which works by inhibiting an enzyme that degrades an antioxidant called glutathione. Glutathione works by neutralizing reactive oxygen species and thus, it is believed that it may decrease oxidative stress by blocking glutathione degradation.
Acivicin was also reported to have blocked microglial activation in cell culture experiments and prevented the development of symptoms in a mouse model of MS. In mice with well-established, chronic progression of the disease, treatment with acivicin also suppressed their symptoms.
“This was exciting because it told us that oxidative stress may be a key driver in maintaining the clinical severity of MS, not just in the initial nerve damage,” mentioned Ryu.
The scientists noted that acivicin itself may not hold promise as a therapy for MS due to its potential toxic side effects, however, the discovery has pointed towards the glutathione degradation pathway as a critical player in regulating oxidative stress production in immune cells relevant to MS.
As oxidative stress is a common pathological process in neurological and autoimmune conditions, Tox-seq could aid researchers in investigating mechanisms and finding therapeutic targets for many types as diseases.
“We hope that Tox-seq will open the way to more disease-relevant transcriptomics. Information captured in genes can now be more readily related to a disease process, which will accelerate drug discovery,” concluded Akassoglou.
Sources: Mendiola AS, Ryu JK, Bardehle S et al. Transcriptional profiling and therapeutic targeting of oxidative stress in neuroinflammation. Nat. Immunol. doi:10.1038/s41590-020-0654-0 (2020) (Epub ahead of print); https://gladstone.org/news/toxic-cell-atlas-guides-new-therapies-neurodegeneration